LINOSA AND ITS ROCKS 



31 



Or. 



Ab. 



An. 



Ne. 



Di. 



Hy. 



01., 



Mt. 



II.. 



Hm 



16 



50 



116 



99.99 



lb. Norm of la, gray tuff of Monte Levante. 



116. Norm of Ila, yellow tuff of Monte Pozzolana. 



From these figures the gray tuffs, or rather the original rock on 

 the assumption that the recalculated analysis correctly represents it? 

 falls in the subrang andose (II. 5. 3. 4), while the original rock of the 

 yellow tuff would fall in beerbachose (II. 5. 3. 5), but so close to the 

 border of the docalcic order that it is nearly in hessose (II. 5. 4. 4-5)5 

 so that it should be called a hessose-beerbachose (II. 5. 4-3. 4-5)- 

 The most salient feature about these magmatic positions is the fact 

 that, differing from the fresh lavas, these tuffs are in the dosalane 

 class, not the salfemane. This position is brought about, in part by 

 the high alumina, which increases the amount of normative anorthite, 

 in part by the greater amount of ferric oxide relative to ferrous, and 

 by the lower magnesia, both of which decrease the amounts of femic 

 minerals. But apart from these differences of class, the rocks of the 

 tuffs would seem to have been closely concordant, as far as the order, 

 rang, and subrang are concerned, with the later lavas, and they were 

 probably originally also in the salfemane class. 



GENERAL PETROLOGICAL RELATIONS 



Character and succession of the lavas. — The figures shown by the 

 analyses, and the facts in regard to the relative ages of the various 

 volcanic cones, presented in the preceding pages, furnish us with the 

 data for a brief discussion of the general characters of the Linosa 

 lavas, and their order of succession. For convenient reference all of 

 the analyses of the lavas are repeated here, those of the tuffs being 

 omitted, as their original characters are so obscured by weathering 

 as to render these analyses of httle use. There are also given in the 



